Sound absorbing devices of many types have heretofore been proposed. Such prior art devices have relied upon sound absorbing characteristics of materials employed in the device, the geometrical arrangement of a plurality of structural elements, which structural elements alone have no particular advantageous sound absorbing properties, and combinations or hybrids of sound absorbing materials and geometrical arrangements of structural elements.
Sound absorbing materials do not function efficiently at low acoustical frequencies, do not normally have structural strength by themselves sufficient for use in many applications, are readily contaminated in many environmental conditions and are difficult to clean. Geometrical sound absorbing structures or devices, on the other hand, as a normal matter, do not suffer from the disadvantages of sound absorbing materials. However, geometrical sound absorbing structures previously proposed have been relatively complicated and expensive in comparison to cost of sound absorbing materials for comparable sound absorbing performance. This factor is primarily attributed to the required structure and the frequent intervals at which structural elements of features must be reproduced and the resultant cost of materials, machinery, processes, and labor in forming such structures.
Perhaps the oldest known geometrical sound absorbing device is the resonant cavity which is accessible by sound waves through restricted openings, suitably sized and arranged. Such structures typically comprise a network of elongate cellular structures accessible at one end by sound waves through an admittance area of prescribed impedance. The other ends of the cellular structures are terminated by some type of acoustically reflective barrier. By proper control of the admittance of sound waves and geometry of the cellular structures, air or other fluid within the cells can be caused to resonate and, thus, dissipate the energy of the sound waves, largely through viscous losses. Such resonators have very high sound absorbing capacity in a limited frequency band. A plurality of such resonators may be tuned for different frequencies to provide sound absorption over a broad band of frequencies. However, it will be appreiciated that the level of absorption over the band will not be as high as when a plurality of resonators is tuned to a particular frequency of interest. Permeable sound absorbing materials can be provided within all or a portion of the cells to increase the absorption, particularly at the higher frequencies. Some typical prior art devices employing the resonant cavity concept are disclosed in British Pat. Nos. 733,329 and 822,954 and U.S. Pat. Nos. Re. 22,283; 2,887,173; and 3,353,626. More recent and improved prior art geometrical sound absorbing structures based upon resonant cavities have been proposed by Leslie S. Wirt in U.S. Pat. Nos. 3,913,702; 3,831,710 and 3,734,234.
In geometrical sound absorbing structures of the resonator type wherein the resonators have uniform cross-sectional areas throughout their length, it has been determined that at frequencies for which the length of the resonator equals an odd multiple of quarter wavelengths, a resonance or near resonance occurs and good sound absorption is obtained. At frequencies for which the length of the resonator is an even number of quarter wavelengths, an anti-resonance occurs and poor sound absorption is obtained. The frequency at which such a resonator is tuned can be modified by modification in geometrical aspects of the resonator and/or the acoustical impedance of the admittance area to the resonator.
Upon detailed consideration of the prior art structures, it will be appreciated that one of the two distinct approaches has been employed in defining the admittance area for the sound wave into the resonator. One approach has included the use of a facing sheet for the resonator formed from an impermeable material but having minute perforations small in comparison to the other dimensional aspects of the resonator and the wavelengths of the frequencies to be absorbed. The other approach has included forming the resonators of a cellular structure having mutually perpendicular cross-sectional dimensional ratios of unity or near unity with each of the dimensions being relatively small in comparison to the wavelengths of the frequencies to be absorbed. Such constructions find their basis in the odd multiple quarter wavelength therory of resonance wherein the sound waves are received and propagated as plane waves longitudinally within the resonator.
It is the purpose of the present invention to provide a geometrical sound absorbing structure having good sound absorbing characteristics that does not require limitations included in the prior art devices previously described.